Not Applicable
The present invention relates generally to tunable lasers and, in particular, to a widely tunable laser having a tunable multi-mode interference section.
Tunable lasers play an important role in the dense wavelength division multiplexing (DWDM) systems that form the backbone of today""s optical communication network. In particular, they are generally regarded as being the preferred transmitters of future long distance and metropolitan optical systems. In this regard, the term xe2x80x9ctunable laserxe2x80x9d is typically applied to a single-wavelength laser, wherein the wavelength can be varied in a controlled manner. For example, laser diodes are frequently used to fabricate tunable lasers.
A laser diode is a p-n junction semiconductor that is operated with an input current sufficiently high to produce gain to compensate optical and mirror losses. Typically, two cleaved facet ends define the laser cavity, and serve as mirrors to reflect light back and forth throughout the xe2x80x9cgain mediumxe2x80x9d. In one embodiment, wavelength selectivity is imparted to the laser with a Bragg grating(s), or modified Bragg grating(s). The Bragg grating(s) are positioned near one or more of the cleaved facets ends, as for example found in distributed Bragg reflector (DBR) lasers, or are distributed throughout the laser cavity, as for example found in distributed feedback (DFB) lasers. Wavelength tuning is generally accomplished by changing the optical path length of the cavity and/or modifying the Bragg wavelength of the grating. In either case, this is most conveniently performed by changing the refractive index, which is optionally achieved by current injection, voltage bias and/or altering the temperature.
Referring to FIG. 1, there is shown a prior art three-section DBR tunable laser. The tunable laser 100 includes an optical gain section 101, a phase control section 102, and a tunable Bragg grating section 103. A first current source 104 pumps the gain section 101 to generate optical gain, a second current source 105 injects carriers to adjust the optical path length of the cavity such that the laser resonant frequency approximately matches the peak of the Bragg grating, and a third current source 106 controls the reflectivity peak by changing the effective refractive index neff of the Bragg grating 103. Light is output on the right hand side of the device, as indicated by the arrow. A wavelength tuning range of about 6-12 nm is achieved using this design.
Widely tunable lasers, which for example have a tuning range greater than about 30-40 nm, present an inherently difficult problem because they cover a wider tuning range than the material tuning range. At the same time, very precise wavelength selection and high stability is required. In order to extend the tuning range of diode lasers, various modifications to DBR lasers have been proposed.
In the sampled-grating distributed Bragg reflector (SG-DBR) laser, the Bragg grating is replaced with a sampled grating. A sampled grating is essentially a modified Bragg grating, wherein grating teeth are periodically removed along the length of the grating (i.e. the grating is xe2x80x9csampledxe2x80x9d). Sampled gratings exhibit a comb-like reflectance spectrum, wherein the spacing between comb peaks is inversely proportional to the period of the sampling. FIG. 2 is a schematic diagram of a prior art SG-DBR. The SG-DBR includes a front sampled grating section 202, a gain section 204, a phase section 206, and a back sampled grating section 208. The sampled gratings 202a, 208a serve as wavelength selective mirrors that form the laser cavity. In accord with the Vernier effect, the front and back sampled gratings are sampled at different periods such that the comb pitch of the comb-like reflectance spectra are different, and such that only one peak from each of the two different reflection spectra can overlap. Tuning the laser is accomplished by adjusting the refractive index within each of the front and back sampled gratings, such that all of the reflection peaks simultaneously move until the closest reflection peak of each grating is aligned at the desired channel, and lasing occurs. In other words, the SG-DBR laser overcomes the aforementioned difficult problem by using a tuning mechanism with two degrees of freedom. Instead of using one knob with really fine control, two knobs with coarser control are used in tandem.
In the grating-assisted coupler with sampled rear reflector laser, also referred to as the Grating assisted co-directional Coupler with Sampled grating Reflector (GCSR) laser, the aforementioned difficult problem is overcome by providing a first knob to, coarsely define the wavelength range of the desired channel and at least a second knob for fine tuning to the desired channel. The prior art GCSR, which is illustrated in FIG. 3, includes a gain section 302, a coupler section 304, a phase section 306, and a back sampled grating section 308. Each of the sections is controlled by injecting current thereinto. In particular, injecting current into the coupler section 304, the back sampled grating section 308, and the phase section 306 corresponds to coarse, medium, and fine tuning, respectively. Accordingly, the control is easier and more straight forward than in the SG-DBR laser. Unfortunately, the grating assisted coupler is associated with a more complicated fabrication, since there are two waveguides in the transverse direction. Furthermore, the ideal coupler length is highly fabrication dependent.
It is an object of the instant invention to provide a tunable laser having separate coarse, medium, and/or fine tuning.
It is a further object of the instant invention to provide a tunable laser that is relatively simple to control and easy to fabricate.
The instant invention relates to a tunable laser having a tunable multi-mode interference (MMI) section. Preferably, the MMI section is tuned by current injection into predetermined regions thereof. Advantageously, this configuration provides separate coarse, medium, and fine-tuning. Moreover, this configuration is relatively simple and easy to fabricate.
In accordance with the invention there is provided a tunable laser comprising: a gain section including an active waveguide and a first current injector, the first current injector for injecting current into the active waveguide so as to provide optical gain; a phase section including a transparent waveguide and a second current injector, the second current injector for injecting current into the transparent waveguide so as to produce a refractive index change therein, the transparent waveguide optically coupled to the active waveguide; a multi-mode interference section including a multi-mode waveguide and a third current injector, the third current injector for injecting current into the multi-mode waveguide so as to produce local refractive index changes therein, the multi-mode waveguide optically coupled to the transparent waveguide; and a sampled grating section including another transparent waveguide and a fourth current injector, the other transparent waveguide coupled to a sampled Bragg grating, the fourth current injector for injecting current into the sampled Bragg grating so as to produce a refractive index change therein, the other transparent waveguide optically coupled to the multi-mode waveguide.
In accordance with the instant invention there is provided a tunable laser comprising: a substrate; an optical waveguide disposed on the substrate, the optical waveguide extending from a port on a front facet of the substrate towards an opposing end of the substrate; a sampled Bragg grating disposed near the optical waveguide proximate the opposing end, the sampled Bragg grating for reflecting predetermined wavelengths and forming a laser cavity with the front facet; and an active region disposed in the optical waveguide proximate the front facet, the active region for providing optical gain within the laser cavity, wherein the optical waveguide includes a multi-mode interference region disposed between the active region and the sampled Bragg grating, the multi-mode interference region for passing a range of wavelengths such that substantially only one of the predetermined wavelengths is output the port on the front facet, the range of wavelengths being variable in dependence upon an applied current to the multi-mode interference region.
In accordance with the instant invention there is further provided a method of tuning a laser having a gain section, a phase section, a multi-mode interference section, and a sampled Bragg grating section, the method including the steps of: injecting current into a multi-mode interference waveguide disposed within the multi-mode interference section, the current injected in at least one predetermined location selected in dependence upon a desired tuning direction and with a magnitude selected in dependence upon a desired output wavelength of the laser.